1. Field of the Invention
The present invention relates to the field of disk drives. More particularly, the present invention relates to a microactuator for fine positioning a read/write head of a disk drive and a method for making the microactuator.
2. Description of the Related Art
FIG. 1 shows a high RPM disk drive 10 having a two-stage, or piggy-back, servo system for positioning a magnetic read/write head (or a recording slider) over a selected track on a magnetic disk 11. The two-stage servo system includes a voice-coil motor (VCM) 13 for coarse positioning a read/write head suspension 12 and a microactuator, or micropositioner, for fine positioning the read/write head over the selected track. FIG. 2 shows an enlarged exploded view of the read/write head end of suspension 11. An electrostatic rotary microactuator 14 is attached to a gimbal structure 15 on suspension 11, and a slider 16 is attached to the microactuator. A read/write head 17 is fabricated as part of slider 16.
While a disk drive having a two-stage servo system can achieve a very high track density in a cost-effective manner, certain issues must be resolved so that such a two-stage servo system is practical for high volume production. For example, when a piggy-backed microactuator is used for fine positioning a read/write head (or a recording slider), the wires connected to the recording head (or the recording slider) act as springs that oppose the forces generated by the microactuator, thus varying the microactuator frequency response. The variations caused by the wires are non-reproducible from microactuator device to microactuator device. Further, conventional wiring attachment techniques restrict bonding pads associated with a microactuator to be at certain locations, while simultaneously restricting the microactuator to have certain dimensions. Further still, conventional wiring attachment techniques add undesirable mechanical stresses to a microactuator structure during the wire attachment process.
Large electric fields extend between the stationary and movable electrodes and when the electrodes are exposed, and the strong fields generated during operation make dust more likely to collect on the electrodes possibly causing short-circuit failure and/or jamming of the moving structure of the microactuator. Moreover, when high-frequency signals are applied to exposed electrodes, such as in a sensing circuit, cross-talk interference and EMI to near-by circuits are potentially generated.
When a flexure-type microactuator is used to fine position a read/write head (or a recording slider), the microactuator experiences forces along a vertical direction that is normal to the rotational plane of the microactuator. The forces can be large enough to exceed the flexure strength of the microactuator, resulting in permanent deformation or breakage of the flexure. For example, a negative air-bearing force, which forces the recording slider away from the microactuator substrate during unloading, can be greater than 10 g. The recording slider is also exposed to horizontal forces caused by contact friction and air drag, although these particular forces are relatively minor in comparison to the vertical forces experienced by a microactuator. Further, the recording slider may hit asperities on the surface of the disk that causes the slider to rotate around suspension pivot points. All of the slider motions may apply repetitive stresses to the microactuator flexure.
Additionally, it is important for the microactuator to be area-efficient so that the area and mass of the microactuator are minimized. The costs associated with fabricating a microactuator are typically directly proportional to the area of the microactuator, so the smaller the area, the less expensive the microactuator will be. The mass of the microactuator, which is attached to the tip of the suspension, has large effect on the dynamics of the coarse actuator (VCM). Consequently, minimizing the mass of a microactuator favorably impacts the dynamics required of the coarse actuator.
There are two desirable design constraints for designing the comb-electrodes and the beams of a microactuator for producing an area-efficient microactuator. The comb-electrodes are the part of the microactuator structure that produces the electrostatic driving force. The electrodes, also referred to as fingers, are attached to the beams, or electrode branches, that are fixed to both the stationary part and the movable part of a microactuator. For the first design constraint, the longitudinal axis of each finger is preferably oriented at the base of the finger where it attached to a branch to be perpendicular to an imaginary line that connects the base of the finger to the center of the rotation of the microactuator. By so doing, the electrostatic force produced between the electrodes generates a pure torque. For the second design constraint, each electrode branch of the stationary part of a microactuator must be separated from each adjacent branch of the stationary part of the microactuator by a minimum distance along the length of the branches. Similarly, each electrode branch of the movable part of a microactuator must be separated from each adjacent branch of the stationary part by a minimum distance along the length of the branches.
Conventional microactuator designs using a "radial" or a "spoke-like" branch arrangement satisfy the first design constraint. FIG. 3 shows a conventional arrangement 30 of a flexure-type rotary microactuator having stationary branches 31 and movable branches 33. Fingers 32 and 34 are attached to branches 31 and 33, respectively, so that a longitudinal axis of each finger is oriented perpendicularly to an imaginary line that connects the base of the finger to the center of rotation 35. Nevertheless, the separation distance between branches 31 and 33 increases with increasing distance from the center of rotation of the microactuator, resulting in a relatively low area-efficiency.
To make a two-stage servo system for a high-RPM, high-track density disk drive practical for high volume production, what is needed is a wiring technique that provides a precisely-defined spring stiffness and allows contact pads to be placed at any convenient location on a magnetic recording slider. Further, what is needed is a way to shield the electrodes so that dust is not likely to collect on the electrodes and cross-talk interference and EMI to near-by circuits is minimized. Further still, what is needed is a limiter design that confines the stresses applied to a microactuator to low levels, thus avoiding any deformation of a microactuator flexure. Lastly, what is needed is a rotary microactuator having an area-efficient design.